1. Field Of The Invention
The present invention relates to a process for preparing a polyolefin. More particularly, it relates to an improved process for preparing a polyolefin having improved hue.
2. Description Of The Prior Art
Heretofore, several processes for preparing a polyolefin which use a Ziegler catalyst have been known. These processes can be classified into those using an inert solvent and those using no solvent. The former processes (using an inert solvent) can be further classified into those employing either a solution polymerization system or a slurry polymerization system, depending upon the conditions of polymerization such as temperature, types of inert solvent, concentrations of polyolefin, and the like. The latter processes (using no solvent) can be further classified into those employing vapor phase polymerization, solution polymerization or slurry polymerization of the olefin monomer, depending upon the conditions employed such as temperature and pressure.
After the polymerization of an olefin by any of these processes, the polyolefin products can be obtained by one or more further treatment steps. After a solution polymerization using an inert solvent, these subsequent steps include interruption of polymerization, separation of the unreacted olefin, separation of catalyst residues, precipitation of solid polyolefin, separation of solvent and drying. After a slurry polymerization using an inert solvent, these steps include interruption of polymerization, separation of unreacted olefin, separation of catalyst residues, separation of solvent and drying. After a vapor phase polymerization using no solvent, the steps include interruption of polymerization, separation of unreacted olefin, and separation of catalyst residues. After a solution polymerization or a slurry polymerization using no inert solvent, these steps include interruption of polymerization precipitation of solid polyolefin, separation of unreacted olefin, separation of catalyst residues and drying. In these treatments, if necessary, other steps such as pelletization can be added or the order of the steps can be changed. From an industrial viewpoint, it is preferred to eliminate certain steps and to combine others. For example, when the polymerization is conducted by using a catalyst having high catalytic activity, the amount of the catalyst residue is so small that the separation of catalyst residues can be eliminated. This results in a very significant industrial advantage. However, conventional polyolefins prepared by using a Ziegler catalyst, especially those prepared without a separation of catalyst residue, possess inferior hue and thermal stability. The inferior hue may be caused by the mutual action of the halogen and the metal compound of the catalyst residue on the polyolefin product or by the mutual action of the catalyst residue and other additives such as a stabilizer.
Consequently, in processes for preparing polyolefins, especially those polymerized in the presence of a Ziegler catalyst having high catalytic activity, there is a great need for improvement of the hue of the products prepared without separation of the catalyst residue.